I came across an unexpected counter to Knights today. Playing a pick up game in my local GW store against a regular AM player with a Stormshadow Super Heavy Tank and a Knight, I managed to scrape up an unoptimised 1850 of Orks to face up to the tank heavy beast opposite.
Luckily I had enough Tankbustas with me to hopefully give my opponent pause for thought, however, it wasn't them that did the most damage.
It was my Lootas.
Whilst the TB's dealt with the horde of Leman Rus' and the SH tank, My Lootas spent every moment blasting chunks off the knight. Chucking out enough Str 7 hits over 2 rounds of firing to knock all but one hull point off the bugger before he got finished off in an astonishingly lucky round of melee by my Morkanaught.
So, Lootas, basically worth their weight in plastic gold. Able to blast troops, light vehicles, Flyers, MC's and now Super Heavies off the table.